A conventional electret condenser microphone is known from Japanese Patent Application “Kokai” No.: Hei. 10-98796. According to this conventional art, sound holes and sound guiding holes are arranged with offset each other so as to prevent intrusion of noise from the outside. Further, for the purpose of rejecting high-frequency noise, a capacitor is interposed between an output terminal connected to a drain terminal and to a source terminal of an FET (Field Effect Transistor) and a ground terminal and also a coil is incorporated in a line from the output terminal. Many other conventional electret condenser microphones have a simpler circuit construction only providing the capacitor interposed between a signal line of the FET and the ground. And, as disclosed by Japanese Patent Application “Kokai” No.: Hei. 7-240424 for example, the conventional electret condenser microphones employ the junction type FET.
The electret condenser microphone having the constructions described above can be readily formed compact, so that it is widely used in a mobile telephone also. When used in a mobile telephone, the electret condenser microphone is to be driven by a battery. Hence, it is desired that the microphone consume as little power as possible. However, in the case of the junction FET, a current, though being small, runs between the signal line and the ground when a voltage is being applied thereto. Therefore, there remains room for improvement in terms of power consumption. In addition, the heat resistance of this junction FET is not very high, so that there is a desire for improvement in the respect of heat resistance also.
In the art of mobile telephone, a conventional digital communication system called GMS (Global System for Mobile Communication) employs frequency of 900 MHz. For high-frequency noise of such frequency, it was possible to reject noise of a particular frequency by allowing the noise to be “drained” from the signal line through the ground by effectively utilizing an impedance drop due to self-resonance of the above-described capacitor or by using a high-pass filter comprising a capacitor in combination with a coil.
However, in the case of a mobile telephone relying on a different system called TDMA (Time Division Multiple Access) for signal transmission, the frequency for its time division is set at the audio frequency. Therefore, there arises the inconvenience that this signal may be received by the signal line, thereby to develop an audio noise thereon. Especially, in the case of the dual-band system employed by more recent mobile telephones, noise rejection has been more complicated since two kinds of high-frequency noise have to be coped with at one time. Furthermore, for the mobile telephone using a high frequency required by the GMS system, there is a need for e.g. a new design change. Hence, there has been a need to cope with noise rejection.
Specifically, the conventional digital communication system called GSM system employs the frequency of 900 MHz. In the case of the dual-band system, however, two frequencies of 1800 MHz and 1900 MHz are employed in combination or more recently two frequencies of 900 MHz and 1800 MHz are employed. Further, in a system called IMT2000, the used frequency is as high as 2 GHz.
An example of the conventional electret condenser microphone is illustrated in FIG. 8. With this conventional construction, the construction includes an electro-acoustic transducer M using an electret element in one of a stationary electrode comprising a front end face or back electrode of a capsule and a diaphragm functioning as an electrode and includes also a junction FET (Tr). The electro-acoustic transducer M is electrically coupled to a gate terminal G of the FET (Tr), a drain terminal D of the FET (Tr) is used as a signal line L and capacitor Con is incorporated between the signal line L and a ground (earth).
With the electret condenser microphone having this conventional construction, by appropriating setting a capacitance of the capacitor Con so that the impedance value of the capacitor Con may drop to its minimal at the particular frequency (900 MHz) employed by the GSM system (specifically, as graphically shown in FIG. 6 as “conventional product”), it is possible to reject noise by allowing high-frequency noise on the signal line L to be drained to the ground. However, even with this construction capable of rejecting high-frequency noise, if the high-frequency noise affects the construction in a cycle corresponding to the audio frequency as is the case with the TDMA system, there often develops on the signal line L an audible noise which has a similar waveform to that demodulated from this high-frequency noise. In particular, when the dual-band type mobile telephone employs the conventional electret condenser microphone, only one of the two kinds of high-frequency noise employed for communication can be rejected. Hence, there remains room for improvement in this respect.
The impedance behavior of the capacitor relative to the frequency can be graphically illustrated as in FIG. 9. From this figure, it may be understood that with the construction having one capacitor, in the dual band system employing the two linds of frequencies of 900 MHz and 1800 MHz, the capacitance of the single capacitor Con can be set so as to reduce its impedance for only one of the two frequencies. It is not possible to reject high-frequency noise of the other frequency for which the impedance is not reduced. As may be apparent from this, with the conventional electret condenser microphone implementing a capacitor alone or using a high-pass filter, it is not possible to cope with noise of these frequencies. Therefore, there has been a need for an electret condenser microphone capable of rejecting noise over a broad band.
The object of the present invention is to construct an electret condenser microphone in a rational manner, which microphone consumes only small power and which is capable of rejecting noise over a broad band.